The invention relates generally to motor vehicle transfer cases and more specifically to a motor vehicle transfer case having a one-way or overrunning clutch and a speed proportional centrifugal clutch disposed in mechanical parallel between the primary and secondary outputs of the transfer case.
Motor vehicle four wheel drive systems have become increasingly sophisticated during the last decade. Such sophistication has been accompanied by improved understanding of vehicle dynamics and the more fundamental requirements of such systems. Better understanding of the fundamentals of vehicle dynamics has revived a focus on less sophisticated mechanical systems. For example, it was once proposed to provide four wheel drive from a primary, typically rear, driveline to a secondary or front driveline through a one-way or overrunning clutch which would engage and transfer torque from the rear driveline to the front driveline whenever the rear driveline attempted to overspeed the front driveline, typically because of a loss of traction of the rear drive wheels. However, due to the characteristics of one-way or overrunning clutches, the system was incapable of providing four wheel drive in reverse. This deficiency was problematic in that the drive system would provide only two wheel drive when, for example, attempting to back out of deep snow or mud or while in reverse when attempting to rock the vehicle.
Also, at speed, a transfer case utilizing a one-way clutch could produce vehicle instability if the foot was lifted off of the throttle and the engine compression braking, which would slow only the rear wheels, created sufficient drag to break traction on these drive wheels.
It is known that the higher the engine speed, the more compression braking the engine creates in an off throttle condition. Since vehicle speed is proportional to engine speed, engine compression braking is also proportional to vehicle speed. The need for torque delivery to the front driveline to overcome the possible instability generating effects of engine braking at the rear wheels is also directly proportional of vehicle speed. Since the speed of the drive shafts of a vehicle also obviously varies with vehicle speed, a device which achieves cross driveline torque transfer, i.e., from the front driveline to the rear driveline in proportion to speed will compensate for the effects of engine braking and improve vehicle performance and stability.